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With this module:

module Foo where

data Foo a = Foo
data Bar a = Bar (Foo a)
    deriving Eq

the HEAD says:

q.hs:5:0:
    Non type-variable argument in the constraint: Eq (Foo a)
    (Use -fglasgow-exts to permit this)
    In the context: (Eq (Foo a))
    While checking the context of an instance declaration
    In the derived instance: (Eq (Foo a)) => Eq (Bar a)

which isn't really the error message that we want, as it's refering to the generated instance.

1. 6 gives the much better:

q.hs:5:5:
    No instance for (Eq (Foo a))
      arising from the 'deriving' clause of a data type declaration
      at q.hs:5:5
    Possible fix: add an instance declaration for (Eq (Foo a))
    When deriving the instance for `Eq (Bar a)'

This example is from tcfail046; tcfail169 is similar and tcfail118 is also a related issue.

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown